A federal grand jury indictment against state Sen. Leland Yee and 28 other defendants adds a conspiracy charge to the seven felony counts the now-suspended lawmaker faces for allegedly promising political favors and gun trafficking to undercover agents in exchange for $62,600 in campaign contributions.

The indictment, made public Friday, covers the same ground as criminal charges filed by federal prosecutors last week, but adds three defendants to the 26 charged previously, along with an additional count against Yee that carries a potential 20-year prison sentence.

The case was assigned to U.S. District Judge Charles Breyer of San Francisco. The defendants are scheduled for arraignment Tuesday.

They will all plead not guilty, according to attorney James Brosnahan, who represents Keith Jackson, Yee's former consultant and fundraiser.

According to a 137-page FBI affidavit released last week, the charges stem from a five-year investigation that initially targeted Chinatown gang leader Raymond "Shrimp Boy" Chow and spread to his associates, including Jackson, a former San Francisco school board president.

The indictment accuses Yee and Jackson of conspiring, between May 2011 and last month, to "defraud the citizens of California of their right to the honest services of State Senator Leland Yee through bribery."

Yee is alleged to have promised political favors to agents posing as donors to his 2011 campaign for San Francisco mayor and his now-abandoned campaign for California secretary of state.

The alleged favors include writing a letter and making a phone call in support of Well Tech, a fictitious software company seeking state contracts; arranging meetings between two state senators and an undercover agent posing as a medical marijuana supplier who sought legislation to help his company; and winning passage of a March 2013 Senate resolution honoring Ghee Kung Tong, a Chinese American association headed by Chow.

In each case, the indictment said, agents contributed $5,000 or more to Yee's campaign. They allegedly made it clear that they were paying for his help - a key element for a conviction of "honest services" fraud.

That charge, common in cases of political corruption, has been interpreted by the U.S. Supreme Court as applying only to outright bribes or kickbacks exchanged for undeserved favorable treatment.

Yee faces six separate fraud charges for each alleged bribe, each punishable, like the conspiracy charge, by up to 20 years in prison.

The San Francisco Democrat, a legislative advocate of gun control, is also accused of conspiring last month with Jackson and Wilson Lim, a Daly City dentist, to import firearms illegally from the Philippines at the behest of an undercover agent who paid Yee $6,800 in cash.

The new defendants in the case are Barry House, charged with taking part in an illegal gun deal with Jackson and with other firearms violations; and Tong Zao Zhang and Zhanghao Wu, accused of trafficking with Chow in stolen cigarettes.

Yee, who was suspended from office by his fellow senators last week, is free on $500,000 bond. Jackson, who faces additional charges of conspiring to smuggle drugs and arrange a purported murder for hire, was released Thursday on $250,000 bail.

Chow, whose charges include $2.3 million in money laundering, is being held without bail.

Yee's attorney, James Lassart, did not respond to a request for comment.